Spin Torque, Tunnel-Current Spin Polarization, and Magnetoresistance in MgO Magnetic Tunnel Junctions

Abstract

We examine the spin torque (ST) response of magnetic tunnel junctions (MTJs) with ultra-thin MgO tunnel barrier layers to investigate the relationship between the spin-transfer torque and the tunnel magnetoresistance (TMR) under finite bias. We find that the spin torque per unit current exerted on the free layer decreases by less than 10% over a bias range where the TMR decreases by over 40%. We examine the implications of this result for various spin-polarized tunneling models and find that it is consistent with magnetic-state-dependent effective tunnel decay lengths. 75.75.+a, The ability of electron currents to transfer spin angular momentum, as well as charge, from one ferromagnetic electrode to another, and hence to exert a significant spintorque on the electrodes (see e.g. refs. 1, 2), provides a powerful new tool for the study of spin transport in electronic structures, in addition to establishing new opportunities for future applications [2,3]. The closely related issue of spin-dependent electron transport in magnetic multilayer structures, both magnetic tunnel junctions (MTJs) [4] and spin valves [5], is of wide-spread interest, both fundamentally and because the importance this phenomena has for information storage [6,7]. A critical aspect of MTJs is the bias dependence of the tunnel magnetoresistance (TMR) [see e.g. ref. 8], which in general, decreases as the voltage bias (V) increases. Currently, there is no consensus as to a microscopic model that accounts for this behavior. Here we report our study of the relationship between bias dependent TMR and spintransfer torque, which is fundamental to understanding both the nature of spin-polarized tunneling at finite bias and spin transfer effects in MTJs [9,10,11]. By making measurements of the thermally activated switching of nanostructured MTJs, we determine the bias dependence of the spin-torque transferred across an MgO tunnel barrier and its relation to the TMR. The spin-torque per unit current is, within 10%, a constant function of V up to ±0.35 V in our devices, in contrast to the TMR which is reduced by >40% at ±0.35 V . This behavior is inconsistent with a decrease in the tunnel polarization factors of the electrodes as described by free-electron tunneling models [12,13,14], or by surface-magnon emission models that substantially decrease the surface magnetization with increasing bias [15]. We find, however, that magneticstate-dependent tunneling decay lengths (effective masses) as theoretically predicted to result in very high TMR in MgO tunnel barriers [16,17,18], are consistent with our results, if we include the effects of our ultra-thin barrier layers having a high density of atomic defects and lower barrier heights than ideal MgO barriers.

Description of current-driven torques in magnetic tunnel junctions

“…Experimental studies by Cornell's group [32,44] usually assumed in interpreting excitations studies [30,31]. Moreover, the very recent article of Sankey et al [32] seems to confirm the fact that the dissipative exchange coupling is quadratic as a function of the bias voltage.…”

Enhancing spin-transfer torque through the proximity of quantum well states

“…7,8 At sufficiently high current densities, the spintransfer torque leads to current-induced magnetization switching ͑CIMS͒. [9][10][11] Reduction of the high critical current for CIMS is necessary for spin-transfer controlled magnetic memories. 12 Double-barrier magnetic tunnel junctions ͑DBMTJs͒ consist of a central metallic layer between two insulating barriers and two FM electrodes.…”

Spin transfer torque switching of magnetic tunnel junctions using a conductive atomic force microscope

“…4, the negative slope of the R versus I curves are due to the expected decrease of magnetoresistance with increasing bias voltage. 19 The noise at currents close to zero was due to voltage measurement uncertainty near zero current and the subsequent division of a finite number by nearly zero in the resistance calculation. Comparison between Fig.…”